This invention relates to a glass window for a vehicle of the kind having a heater extending between bus bars on a surface of the glass, usually the inner surface of the glass when the window is a rear window installed in an automobile. The heater may be an array of heating elements or an electrically conductive heating film on a surface of a single sheet window, or may be embodied within a laminated window construction.
Attempts have been made to provide such a window in which a heater array can also be used as a radio aerial which can be coupled to a radio receiver, but there have been problems of decoupling the radio frequency current from the direct current supplied by the power supply circuit of the vehicle. The direct current circuit is connected to the usual automobile DC power supply, one terminal of which is usually earthed, and which is liable to carry considerable noise signals.
Decoupling circuits suitable for installation in a vehicle have been developed for example as described in GB No. 1,520,030; GB No. 1,600,987 and U.S. Pat. No. 3,484,584. However there have still been problems because the conventional arrangement of a heater array is not particularly suitable for use as a radio aerial. Problems are caused by stray impedances between the heater array and the surrounding metal parts of the car body, in particular the capacitance between the leads which supply heating current to the heater array and the metal parts of the car body which frame the window, especially if the leads are not fixed.
A usual heater array on a rear window of an automobile comprises an array of fine electrical resistance heating elements which extend across the rear window between bus bars down the sides of the window. This array is printed onto the window using a conventional ink containing silver, pigment and glass frit which is printed onto the window and then fired.
It has been proposed to extend the bus bars down the sides of the window and along the bottom of the window for connection to the decoupling circuit. This has not proved satisfactory because of local overheating resulting in failure of a bus bar or damage to the glass, and uneconomic dissipation of power in the bus bars.
It has also been proposed to use a "folded" heater array in which the bus bar at one side is separated into two parts the nearer ends of which are close together and are separately connected to terminals for connection to the decoupling circuit, whilst the bus bar at the other side of the window simply interconnects all the heating elements. The heating elements are thus divided into an upper group and a lower group, with all the elements in each group connected in parallel, and the two groups connected in series across the terminals. This operates satisfactorily as a radio aerial, but the unconventional arrangement of the heater array necessitates the use of thicker conductors employing larger than usual amounts of relatively expensive, silver-containing ink. A disadvantage in some vehicles of having both the terminals near one side of the window is that the decoupling circuit, which has to be close to the terminals, has to be mounted in one of the rear pillars supporting the car roof. This may be difficult because of the bulk of the decoupling circuit, particularly in vehicles with tail gates.